Field of Invention
One or more embodiments of the invention relate generally to the field of digital communications and more particularly, to techniques for generating and adjusting reference voltages and impedances.
Description of Related Art
Processing speeds, system flexibility, and size constraints are typically considered by design engineers tasked with developing computer systems and system components. Computer systems generally include a plurality of memory devices which may be used to store programs and data and which may be accessible to other system components such as processors or peripheral devices. The memory devices, typically, are grouped together to form memory modules such as dual-inline memory modules (DIMMs). Further, computer systems may incorporate numerous memory modules to increase the storage capacity of the system.
Generally, each memory device of a memory module includes one or more off-chip drivers (OCDs) for driving signals off-chip during data transmission and one or more on-die termination (ODT) circuits for terminating the off-chip transmission-line during data reception. Both the output impedance of the OCD and the termination impedance of the ODT are critical to maintaining suitable signal integrity during chip-to-chip communication, as the relationship between these values (OCD impedance and ODT impedance) and the characteristic impedance of the chip-to-chip transmission line will determine both the nominal signal swing and the level of signal-degrading reflections on the line. Because the OCD impedance and ODT impedance are critical, both values are typically made adjustable, which adjustments often take place during a link initialization stage at system power-up.
In pseudo-differential links, such as those adopted by many memory interface standards, a reference voltage (Vref) may also be generated by a reference voltage circuit and provided to the data capture circuitry during data reception. However, variations in supply level and temperature will cause all of these parameters, OCD impedance, ODT impedance, and Vref level to fluctuate or drift with distinct behaviors.